What is the treatment for congelative nonunion of the posterior arch of C1 identified on computed tomography (CT)?

Congenital Nonunion of the Posterior Arch of C1

A congenital nonunion (failure of fusion) of the posterior arch of C1 identified on CT represents a developmental variant that is typically asymptomatic and requires no treatment unless associated with instability, trauma, or neurological symptoms.

What This Finding Means

• Congenital nonunion of the C1 posterior arch is a developmental anomaly where the two halves of the posterior arch fail to fuse during skeletal development, creating a midline cleft or gap 1, 2.

• This condition is distinct from traumatic fractures and represents a normal anatomical variant in most cases, often discovered incidentally on imaging 2.

• The finding can exist in isolation or as part of a bipartite atlas, where both anterior and posterior arches remain unfused 2.

Clinical Significance and When to Worry

• Most congenital C1 posterior arch defects are asymptomatic and stable, requiring no intervention 1, 2.

• Concern arises when this anatomical variant is associated with:

  • Atlantoaxial instability or dislocation 1, 3
  • Concomitant odontoid abnormalities (such as os odontoideum) 1, 4
  • Acute trauma with superimposed injury to the unfused arch 2
  • Progressive myelopathy or neurological symptoms from dynamic compression 1

• In the setting of acute cervical trauma, if a patient with known or suspected congenital C1 arch nonunion presents with upper cervical pain and tenderness, MRI should be obtained to evaluate for superimposed traumatic injury, as edema-like signal in the midline cleft indicates acute injury 2.

Management Approach

For Asymptomatic Incidental Findings:

• No treatment is required for isolated, asymptomatic congenital C1 posterior arch nonunion discovered incidentally on imaging 1, 2.

• Observation only is appropriate in the absence of instability or neurological symptoms 2.

For Symptomatic or Unstable Cases:

• Surgical intervention is indicated when the C1 posterior arch defect is associated with:

  • Atlantoaxial instability causing dynamic myelopathy 1
  • Progressive neurological deterioration 1
  • Failure of conservative management in traumatic injury to the unfused arch 2

• Posterior occipitocervical fusion (C0-C3 or C1-C4) is the treatment of choice for symptomatic cases with instability, as it prevents dynamic compression and further myelopathy 1, 4.

• Alternative fixation techniques may be necessary due to the absent posterior arch, including:

  • C1 lateral mass screws (if vertebral artery anatomy permits) 3, 4
  • C1 posterior arch crossing screws (when sufficient bone stock exists in the remaining arch) 3, 5
  • Occipital fixation extending to lower cervical levels 1, 4

For Acute Traumatic Injury:

• Conservative management with hard cervical collar immobilization for 6 weeks is appropriate for acute traumatic injury to a congenital unfused C1 arch without instability 2.

• MRI confirmation of injury (showing edema-like signal in the midline cleft) guides the decision for conservative versus surgical treatment 2.

Critical Pitfalls to Avoid

• Do not mistake congenital nonunion for acute traumatic fracture on CT imaging—the smooth, corticated margins of a congenital defect differ from the sharp, irregular edges of acute fracture 2.

• Do not dismiss upper cervical pain in a patient with known bipartite atlas after trauma—obtain MRI to evaluate for superimposed injury even if CT shows only the baseline congenital defect 2.

• Recognize that congenital C1 arch defects may coexist with other cervical anomalies (os odontoideum, persistent intersegmental arteries, congenital C2-C3 fusion) that complicate surgical planning and increase risk 1, 4.

• Preoperative CT angiography is essential if surgery is planned, as vertebral artery anomalies are more common in patients with congenital cervical defects and dramatically alter surgical approach 3, 4.